How to remove blur around mirror seam?
The Fix: If you don't need the non-uniform object-level scale for animation, CtrlA &gt; Apply the scale of your object.
Why that's a fix isn't obvious .. your object is scaled equally in X and Y, (though not  in Z). I would have thought normals would be transformed consistently.. it's strange that normals of adjacent faces which previously were equal, are not equal after after this scaling transform. I think we would have to dig into that calculation to figure it out.
The object has an Edge Split modifier with a threshold angle. You can sharpen the object all round by increasing the smoothing threshold in there. (The non-uniform scale makes the face-angles vary around the circumference, even though they all measure the same in object-space, so the effect progresses around the circle, rather than happening all at once)
Edge Split prevents normal interpolation across edges by duplicating them .. it's almost a legacy feature in this context. Probably better to use Auto Smooth, which splits the vertex-per-face normals, rather than the faces themselves. (You can increase the smoothing threshold angle in there, too, for a fix, but the right fix is to apply the scale, unless you need it the way it is.)